On the left and below are woodlice, also known as sow-bugs or slaters, Order Isopoda, Sub-order Oniscidea. In the Crustacea phylum they are the only class that contain species that are wholly terrestrial. Although there are freshwater (see Ascellus below) and marine species. There are over 7 500 species of woodlice in the world, 37 species are native to the British Isles, but there are a few introduced species which can breed indoors only. Most range in size from 5 - 15 mm, although there is a deep sea giant aptly named Bathynomus giganteus, which can reach 42 cm long and 15 cm wide! Most are grey or grey/brown in colour. The animal's whole life revolves around the avoidance of desiccation. Therefore it is active mainly during the night.

On the left you can see the underside of a terrestrial woodlouse, they have 7 pairs of walking legs. The first pair of antennae are usually short, and in terrestrial species they are often vestigial. The second pair of antennae are usually well developed, except in the parasitic species.

The female carries her eggs and young in a fluid-filled pouch beneath her (see Ascellus below left), and it takes about two years for the young to reach maturity.

UK woodlice (and most others) feed mainly on rotting vegetation, and so help to return valuable nutrients to the soil.

They can exude a repellent secretion from their rear end to warn off predators.

Right the woodlouse has just moulted. Unlike insects they do not moult their whole exoskeleton at once, but in two parts. They can moult up to 25 times.

Oniscus asellus (above left) is often found under logs and in compost heaps. Note the lighter patches on its body.

Porcello scaber (top and above right) can tolerate drier places.

And Armadillidium vulgare, the pill bug (right), can tolerate even drier places, also its ability to roll up helps it conserve moisture. It can be found in grassy places, and is often confused with the pill millipede, but can be distinguished by its numerous small rear segments. When fully grown it can be up to 1.8 cm long. The pill bug was once used by doctors - that's where it gets its name- patients had to swallow one whole!

On the left is Porcello spinicornis, a particularly attractive woodlouse. Fully grown it reaches about 12 mm long. It has a black head, black eyes, a dark central stripe with a row of yellow blotches down either side. It can run quite fast, but usually stays still for a while when first disturbed. It is found in wall and buildings especially those with lime-rich mortar. It is very common in north east Scotland and in drystone walls in the Cotswolds.

On the left and below is the freshwater louse or hog louse, Ascellus sp. The drawing on the left shows the underside of a female with eggs (just one set of legs are drawn).